The subject of this invention is an optical modulation component, a Meuller polarimeter and ellipsometer containing such an optical component, a process for the calibration of this ellipsometer, and an ellipsometric measurement process.
In order to measure the magnitudes which are representative of a sample, such as optical, physical, chemical or electrical magnitudes, it is conventional to make use of an ellipsometer. However the field of application of the standard ellipsometer is restricted to non-depolarising, isotropic materials, presenting a plane interface. The ordinary ellipsometer is used to measure non-isotropic materials, but only if they are non-depolarising. Now the current requirements of industrial companies, biologists and chemists frequently concern polarising materials, such as those with rough or speckled surfaces.
A Mueller ellipsometer (or Mueller Matrix Ellipsometer--MME) makes it possible to measure in any medium. In fact the polarisation states of the light can be represented by a 4th dimension vector, which is conventionally the Stokes light vector (S). A description of this can be found in the work of Azzam and Bashara entitled "ellipsometry and polarised light", North-Holland, pp. 55-60. The Stokes vector consists of the I, Q, U and V coordinates, respectively representing the mean intensities of the four different polarisation states. Any medium can then be represented by a real matrix in 4.times.4 dimensions which describes the couplings in intensity of the different polarisation modes of the light, these couplings being provoked by the interaction of the light with the medium. This matrix, known as the Mueller M, includes sixteen coefficients which are generally independent of each other. The Mueller ellipsometer is able to measure the full Mueller matrix of the medium under study. It is therefore particularly applicable to deposits on rough surfaces or measurement on particles, for which other ellipsometric measurement processes are not appropriate.
The arrangements employed for Mueller ellipsometry or Mueller ellipsometers commonly include a light source, a polarisation states generator (or PSG) and a polarisation states detector (PSD) which is also called a polarimeter. A detailed description of these can be found in the article by P. S. Hauge entitled "Recent developments in instrumentation in ellipsometry", Surface Science, vol. 96, pp. 108-140, 1980. In operation, a light beam is emitted from the source, the beam is given polarisation states using the PSG, the beam is sent to the sample to be measured, and the resulting beam is detected using the PSD. Polarisers and quarter-wave blades fixed in the PSG and the PSD are also known to be used, assigning sixteen different configurations to them, in order to measure the Mueller matrix of the sample. This manual technique is difficult to use however, and not very effective.
In "Photopolarimetric measurement of the Mueller matrix by Fourier analysis of a single detected signal", R. M. Azzam, Optics Letters, vol. 2, No.6, pp. 148-150, 1978, a Mueller ellipsometer was proposed in which the PSG and the PSD respectively included two quarter-wave blades revolving at .omega. and 5.omega.. The sixteen coefficients of the Mueller matrix (W)ere then extracted from the first sixteen harmonics of the frequency .omega. of the detected signal. This arrangement had the disadvantage of being restricted to very low frequencies, and to require the use of a "chopper" in order to eliminate the ambient background noise.
In "Mueller-matrix (M)easurement using the four-detector photopolarimeter", R. M. Azzam, Optics Letters, vol. 11, No. 5, pp. 270-272, 1986, Mueller ellipsometers were proposed with a multi-channel PSD and a rotating delay device in the PSG. These devices were also limited to low modulation frequencies, typically necessitating several seconds for acquisition.
In "Measurement of Mueller matrices", R. Anderson, Applied Optics, vol. 31, p. 11, 1992, a single-channel ellipsometer was also proposed which included a high-frequency polarisation modulator in the PSG and the PSD. However several configurations had to be combined in order to get a complete measurement.
In "Measurement of polarised light interactions via the Mueller matrix", Randall C. Thompson et al., Applied Optics, vol. 19, No. 8, pp. 1323-1332, 1980, a high-frequency ellipsometer was envisaged using four elector-optical phase modulators at different frequencies and sixteen synchro-detectors. This arrangement had the disadvantages of being expensive and complicated to implement, in addition to being sensitive to noise since the continuous component of the signal was not modulated.
An article in LEST Technical Report by Stenflo et al., No. 54, XP000645515, 1992, and another in the Optical Engineering Review by Povel, vol. 34, No. 7, XP000517055, pp. 1870-1878, revealed a system for simultaneous measurement of the four Stokes parameters, and its application to polarimetry. The measurement system includes two piezzo-elastic modulators operating at the same frequency, coupled in phase and with an angular shift of 45.degree. in their orientations, a super-achromatic .lambda./4 plate, and a linear polariser, with the modulators, the plate and the polariser placed successively on an optical track.
This system gave rise to problems with synchronisation of the modulators and with control over their coupling. It also had the drawback of being sensitive to noise.